In an LED grown as a layer structure on an insulating substrate, both n- and p-contact electrodes are made from the top side of the chip. Normally they are located near the opposite edges of the light-generating region of the chip. One of them is formed on a lower current-distributing layer exposed by selectively etching away the layers grown on it and the other on an upper current-distributing layer on top of the LED layer structure.
In a typical case, the horizontal distance between the electrodes is much higher than the overall thickness of the vertical layer structure of the LED. In that case, to provide a high current uniformity over the light-generating layer, the sheet resistances of the current-distributing layers should be as low as possible in order to minimize voltage drop along those layers. Particularly, the sheet resistances should be outstandingly smaller than the average vertical resistance through the layer structure (including e.g. resistance due to voltage drop in the p-n junction). However, in most practical cases, said sheet resistances produce substantial voltage drops along the current-distributing layers. This leads to a non-uniform current density resulting possibly in local overheating at the high current density sites. This local overheating can further result in a drop in the device efficiency, as well as in degraded device reliability.
A characteristic parameter relating to the current uniformity is the current spreading length:
            L      s        =                            ρ          vertical                                      ρ                          sheet              ,              top                                +                      ρ                          sheet              ,              bottom                                            ,where ρvertical is the average vertical resistance through the LED structure and ρsheet, top and ρsheet, bottom are the sheet resistances of top and bottom current-distributing layers. In order to avoid the above-described voltage drop along the current spreading layers, the horizontal distance between the electrodes should be lower than the current spreading length. The problem is that, in most practical cases, the horizontal chip dimensions are clearly higher than the current spreading length. Thus, with conventional simple contact pads, said condition relating to the distance between the electrodes can not be fulfilled. One known solution for this problem is using a finger-like, i.e. an interdigitated electrode geometry. With this kind of geometry, the horizontal distances between the electrodes can be adjusted smaller than the current spreading length. However, also this approach has its deficiencies: current density is always highest near the electrodes, particularly at the outer ends of the electrode “fingers”, resulting in a significantly non-uniform current density.